The electroretinographic (ERG) abnormalities associated with diabetic retinopathy will be determined and related to the severity of retinopathy and to other visual function changes (color vision and visual fields). ERG parameters to be measured are the dark adapted b-wave amplitudes in response to an intensity series fit to the theoretical Naka-Rushton equation [(to compute R max (maximum response), K (half-saturation constant), n (slope of response function)], and the ERG oscillatory potentials (amplitude and implicit times). The ability of the electroretinogram and other visual function tests to identify eyes at risk to progress to the more severe retinopathy stages will be tested in a longitudinal 5-9 year study of 250 diabetic patients who are part of the Early Treatment Diabetic Retinopathy Study. The EGR parameters, alone and in conjunction with the Farnsworth-Munsell 100 hue test scores and Goldmann visual field scores at baseline will be related to the subsequent development of proliferative diabetic retinopathy. If these tests prove to have predictive value, they can be used in the future to select patients with high risk characteristics in order to institute earlier or more vigorous medical or surgical therapeutic intervention.